Rotary Hearth Sintering Furnace

ABSTRACT

A rotary hearth sintering furnace composed of a debinding system, a part loading station, a rotary hearth furnace having multiple heating zones, an atmosphere system for maintaining certain atmospheres within different zones of the furnace, an unloader station and a cooling conveyor that are preferably controlled with a single programmable logic controller and operating station for sintering powder metal parts in a minimal amount of space.

CROSS REFERENCE TO RELATED APPLICATION

The present invention claims priority to United States ProvisionalApplication Ser. NO. 60/654,223, filed Feb. 18, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to hearth furnaces, and morespecifically to rotary hearth furnaces used for sintering stainlesssteel/high temperature powdered metal parts.

2. Description of Prior Art

Powder metal is one of four major methods of forming metal, the otherthree being casting, machining, and plastic forming of either hot orcold metal. Powder metal has many advantage over the other threeprocesses. For instance, the labor associated with producing PM parts isgenerally lower than that required using other processes. In addition,close tolerances and unique material properties can be easily achievedusing PM, as can production of intricately shaped parts, such as thoserequiring internal or external splines, gears, knurls, eccentric holes,hidden pockets, and the like. In addition, the material efficiencylowers part costs in that essentially 100% of the material used in thePM process is in the finished part, leaving virtually no scrap.

One traditional drawback of powder metal is its lower density. Advancesin metallurgy, however, have made high density powder metal parts havinga unique binder possible when sintered at temperatures around 2500°Fahrenheit. The density that can be achieved, for instance, is now up toabout 99.7% of the metal's theoretical density.

Conventional process components are sintered at 2150° F. or below. Thisis the upper limit for conventional mesh belt sintering furnaces. Thus,the conventional mesh belt sintering furnaces will not be suitable forsintering operations of the powder metal parts that use a binder thatproduces higher density parts but requires sintering at temperaturesaround 2500° F.

While ceramic belts may be substituted for the mesh belts to permithigher operating temperatures, the ceramic belts have lower loadingcapacity, typically around 6 lbs/sq. ft., which severely limits thethroughput of this type of furnace.

An alternative system is an elongated pusher style furnace wherein theparts are loaded in one end of the furnace, pushed through the furnace,and unloaded at the opposite end. This type of furnace can operate atthe required temperatures and can process more pounds per hour than thebelt system, but do have other drawbacks. For instance, the pushersystem requires setter plates on which the parts will be placed andpushed through the furnace. These plates add significantly to the costof operation, are prone to breakage from thermal cycling and handling,and can misalign and pile up while being pushed through the furnace. Theplates also require a return system to bring them back to the load endof the furnace.

OBJECTS AND ADVANTAGES

It is therefore a principal object and advantage of the presentinvention to provide a rotary hearth sintering furnace for sintering P/Mparts.

It is an additional object and advantage of the present invention toprovide a P/M sintering furnace system that is can efficiently handle alarge throughput of parts in a minimal amount of space.

Other objects and advantages of the present invention will in part beobvious, and in part appear hereinafter,

SUMMARY OF THE INVENTION

In accordance with the foregoing objects and advantages the presentinvention provides a rotary hearth sintering furnace essentiallycomprising a debinding system, a part loading station, a rotary hearthfurnace having multiple heating zones, an atmosphere system formaintaining certain atmospheres within different zones of the furnace,an unloader station and a cooling conveyor. The system is preferablycontrolled with a single programmable logic controller and operatingstation.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood and appreciated byreading the following Detailed Description in conjunction with theaccompanying drawings, in which:

FIG. 1 is a top plan view of the sintering furnace system using platesto carry the parts;

FIG. 2 is a top plan view of the sintering furnace system, wherein theparts are processed without plates;

FIG. 3 is a cross-sectional view of the load chamber and furnace takenalong section line 3-3 of FIG. 1; and

FIG. 4 is a cross-sectional view of the load chamber and furnace takenalong section line 4-4 of FIG. 2.

DETAILED DESCRIPTION

Referring now to the drawings, in which like reference numerals refer tolike parts throughout, there is seen in FIGS. 1 and 2 a powder metal(“P/M”) sintering system, designated generally by reference numeral 10,and essentially comprising a rotary hearth sintering furnace 12, ahearth loading chamber 14 in communication with furnace 12, a hearthunloading chamber 16 in communication with furnace 12, a debind conveyor18 in communication with load chamber 14, and a cooling conveyor 20 incommunication with unload chamber 16. P/M sintering system 10 is adaptedto perform automated high temperature sintering of P/M parts 22 withhigh efficiency and high throughput. Furnace 12 is designed to operateat temperatures suitable for sintering P/M parts, for example, up to atleast about 2500° F. It is contemplated, however, that the furnace becapable of operating at temperatures of up to about 3000° F. in order tobe suitable for high temperature sintering applications other than forP/M parts.

The sintering process begins at debind conveyor 18 where P/M parts areloaded at its front end. Conveyor 18 runs along a longitudinal axis A-Awithin a sealed compartment in which a first, controlled atmosphere willbe maintained at a temperature of about 1100° F. to 1550° F. At thistemperature, it is typically about a 20 minute process for the debindingto be complete.

P/M parts typically contain an organic binder that holds the parttogether, and this binder is preferably burned off prior to entering thefurnace for sintering. The first, controlled atmosphere for burning offthe organic binder is typically 100% endothermic, Nitrogen-Hydrogen,Nitrogen bubbled through water, and a rich burning natural gas burnerfiring into the debind chamber. The precise type of atmosphere used,however, is dependant upon the type of binder being processed and thedesired properties, the appropriate selection of which is known to oneskilled in the art.

It should be noted that debinding could be performed within the furnaceif the furnace was large enough to accommodate the heat and atmospherezone necessary to complete the debinding process. The debinding couldalso be done off-line relative to the sintering process, but this, ofcourse, increases the amount of part handling necessary to complete thesintering process.

Once coming off debind conveyor 18, the parts either pass on to settertrays 24 positioned within loading chamber 14 or are moved directly ontoa conveyor 26 positioned within chamber 14 that will convey the partsinto furnace 12. Typically, parts weighing over 2 pounds will godirectly into furnace 12, while parts weighing less than 2 ponds aremore efficiently handled by loading onto tray 24 and then conveying thetray into furnace 12. It is possible, however, to load all types ofparts onto a setter tray or all parts directly into the furnace withouttrays. If trays are used, they remain in the furnace until they areloaded and unloaded. By keeping the trays at the furnace temperature, asopposed to room temperature, the efficiency of the sintering process isimproved as there is no appreciable heat transfer between the trays andthe parts that are placed thereon.

Relative to the loading mechanisms and with reference to FIGS. 3 and 4,a two-axis loading system may be employed. When plates 24 are used, aplate loader 28 extends in a horizontal plane adjacent the bottom ofchamber 14 and accepts setter plates 24 transported from the unloadchamber 16. PLC (programmable logic controllers) controlled hydrauliccylinders 30, or equivalent systems such as servo-controlled systems,move setter plates 24 from the plate loader to the debind conveyoropening that is positioned in a horizontal plane vertically above theplane in which the plate loader extends, and where a part loader 32moves parts from debind conveyor 18 onto setter plates 24. Once setterplates 24 are loaded, the controller opens the furnace door and partloader 32 automatically moves the plates into furnace 12. When platesare not used, the parts simply come in from the debind conveyor 18 andare raised from a part loader (the same as plate loader 28) onto theconveyor that will take them into furnace 12.

At the entry point of furnace 12, an atmosphere pressure blowerintroduces the desired atmosphere. The atmosphere preferably consists of75% Hydrogen/25% Nitrogen to 100% Hydrogen. Furnace 12 can be aconventional rotary hearth with upper and lower refractories 34, 36,respectively, and a hearth 38 that rotates about the central axis offurnace 12. Preferably, a servomotor drive system, oil lubrication andcooling systems, and a drive system comprising a large diameter thrustbearing with gear toothed outer race, a pinion gear, and a doublereduction gear reducer, all controlled by the servomotor are employed,the arrangement of which would be known to one of ordinary skill in theart.

The oil lubrication and cooling system circulates oil through the base40 of furnace 12 to cool the base and from there migrates thorough thehearth and provides lubrication to the hearth bearing. A self containedpump unit cools and filters the oil in the system. The oil is gravityfed from furnace 12 to the pump unit where it is filtered, cooled, andpumped back into the base of the furnace.

The setter plates 24 are designed to be preferably about 3 times as longas wide and be placed onto hearth 38 with their longitudinal axisaligned with the radial axis of furnace 12. The rack system is designedto have plates 24 be keyed into hearth 38 and be stackable. The setterplates 24 and fixturing system that key them into hearth 38 will be madefrom refractory or ceramic material capable of withstanding thesintering temperatures and hydrogen atmosphere maintained within thefurnace.

After plates/parts are loaded onto hearth 38, they begin theirrotational travel around furnace 12. The first 180 degrees of travel arein a series of ramped heat zones 40 that are maintained in a hydrogenatmosphere at up to about 2550° F. Depending on the P/M part, the numberof heat zones can be adjusted to ramp up or down in temperature asquickly or as slowly as necessary. In addition, a series of atmosphereports 42 are positioned at predetermined positions around the furnace 12to provide a consistent, positive flow of the desired atmosphere,preferably hydrogen.

After the parts have traveled at least 180 degrees of the way aroundfurnace 12, they enter a cool down zone 44 that gradually reduces thetemperature to which the parts are directly exposed prior to exitingfurnace 12.

After the parts have revolved around furnace 12 for about 324 degrees,they are unloaded from hearth 38 and into unload chamber 16. A doorseparating furnace 12 from unload chamber 16 receives a signal from thecontroller that a plate 24 (or parts) are positioned for movement intochamber 16 and is opened and then closed as soon as the plate/parts areappropriately moved out of the furnace. Unload chamber 16 is virtuallyidentical to load chamber 14, containing all the same elements(designated with the same reference numerals except for the addition ofa “′” sign on the drawings.) As opposed to the parts being introducedinto load chamber 14, however, the parts are passed from the unloadchamber 16 onto an unload conveyor that takes them through a coolingchamber 50 for a predetermined distance. The cooling is preferablyeffected with a forced gas convection cooling system with the conveyor20 riding on a water jacket. At the exit of cooling chamber 50, thesintered P/M parts are taken for further processing.

1. A rotary hearth sintering furnace for treating articles, comprising:a. a hearth having a central axis, an inlet, an outlet, a plurality ofheat zones, and a conveyor mounted for rotational movement about saidcentral axis through all of said heat zones; b. a load chamberpositioned adjacent said inlet; c. an unload chamber positioned adjacentsaid outlet; d. a debind compartment positioned in communication withsaid load chamber; and e. a cooling chamber positioned in communicationwith said unload chamber.
 2. The rotary hearth sintering furnace ofclaim 1, further comprising a plurality of ports spaced about andadapted to introduce a predetermined atmosphere into said hearth.
 3. Therotary hearth sintering furnace of claim 1, further comprising a waterjacket positioned in proximity to said cooling chamber.
 4. The rotaryhearth sintering furnace of claim 1, wherein said debind compartmentincludes a conveyor movable along a longitudinal axis that is adapted tocarry the articles into said load chamber.
 5. The rotary hearthsintering furnace of claim 1 wherein said cooling chamber includes aforced gas convection cooling system.